Method for renewing grinding wheel surfaces and disk and machine for carrying out said method

ABSTRACT

Method for renewing grinding wheel surfaces in a machine for grinding ophthalmic lenses, comprising one or more diamond wheels (2,3,4,5), gripping members (7,8) capable of receiving an ophthalmic lens, and control. Once wheel wear is detected, the surface is renewed using an abrasive disk (9) fitted to the grinding machine in place of a lens in the gripping members. The invention also concerns a disk (9) for carrying out the method, with at least the peripheral surface consisting of abrasive material. A machine for gripping ophthalmic lenses and carrying out the method is also described.

BACKGROUND OF THE INVENTION

The present invention relates to a method for renewing the abrasivesurface of grinding wheels. The method is particularly suitable forrenewing the surface of wheels for grinding machines used in theophthalmic field and comprising one or several diamond grinding wheels,gripping means designed to receive an ophthalmic lens, and controlmeans. The invention also relates to a disk and a machine for carryingout the method.

Grinding machines are used in the ophthalmic field for trimming,bevelling or grooving ophthalmic lenses.

A trimming operation consists in adapting the generally circular contourof an ophthalmic lenses to the contour of the frame designed to receiveit. It consequently consists in removing part of the material of theperiphery of the lens.

Beveling of an ophthalmic lens, which is carried out after trimming,enables the cross-section of the peripheral edge of the lens to beadapted so that it fits into the groove which the spectacle frameusually incorporates. Beveling of the lens consists consequently inproviding a generally triangular-shaped bevel or rib on the peripheraledge of the lens.

A grooving operation on an ophthalmic lens may also be carried out on alens after it has been trimmed. This enables the cross-section of theperipheral edge of the lens to be adapted so that it can be mounted incertain types of frame. In effect, some frames do not include a grooveover the whole internal perimeter of the parts designed to receive thelens; rather, it sometimes happens, for example, that the frame onlymatches the top part of the lens whereas the lower part of the lens isheld by a transparent wire. In such cases, it is necessary to provide agroove or a channel which is most frequently triangular or semi-circularin shape, on the peripheral edge of the lens.

Most frequently, these three operations are carried out on one and thesame grinding machine, fitted with a train of grinding wheels. Suchmachines are known and available commercially.

Grinding machines of this type typically comprise one or several diamondgrinding wheels frequently coupled together to constitute a train ofgrinding wheels, gripping means designed to receive the ophthalmic lensto be machined, and control means. The control means are designed tocontrol the rotation of the grinding wheel and the gripping means whichhold the lens; the control means additionally control the relativedisplacement between the lens and the grinding wheel.

Such a grinding machine generally includes a sensing finger which isdesigned to follow a template corresponding to the particular shape ofthe lens. It may additionally, if appropriate, store the various shapesa lens may be desired to have in memory, and in this case, the sensingfinger most frequently follows the contour of a disk.

Such a grinding machine generally operated in the following manner:

Initially a trimming wheel cuts out the lens to be machined after whicha beveling or grooving wheel provides the bevel or groove in the cut-outlens.

Such machines and their operation are more particularly described inFrench patent number 2,543,039 in the name of the presentapplicant/assignee.

Known grinding machines and more particularly the one described in thesaid patent, operate in a highly satisfactory manner. They however donot provide a satisfactory solution to the problem of wear of thegrinding wheel.

On such grinding machines, diamond grinding wheels are used. Thesegenerally consist of a disk, the periphery of which is covered withlayer consisting of a binder incorporating fine diamond particles. Thediamond layer can be produced by electro-erosion or by sintering. Duringrepeated machining operation on lenses by the grinding machines, thegrinding wheel surface becomes progressively more dull and loses itsabrasive power. The loss is due to the diamond particles in contact withthe lens to be machined gradually getting worn down. It is obvious thatalthough the phenomenon is described for the case of diamond grindingwheels, it also occurs with other types of grinding wheel.

The wear of the grinding wheel may, under certain conditions, becompensated by a self-sharpening effect of the wheel on the glass to bemachined. During machining, the ophthalmic lens being ground wears downthe grinding wheel and thus partially renews the surface thereof.Nevertheless, such self-sharpening does not provide a satisfactorysolution to the problem of wear of grinding wheels. Firstly, ophthalmiclenses are now constituted of very differing glasses having varyinghardnesses which leads to poor self-renewal of the abrasive surface. Inparticular, certain glass-plastics composites may form a fine film onthe wheel, the effect of which is to prevent or slow down self-renewal.Secondly, self-sharpening generally involves a deleterious modificationto the shape of the wheel surface. Such self-sharpening does not in factwear down the wheel uniformly and, if for example self-sharpening hasoccurred with a particular series of lenses, the resulting shape of thegrinding wheel will doubtless have become modified to match the shape ofthe lenses. This consequently involves a risk of damaging lenses of adifferent shape during subsequent machining. This problem is obviouslycrucial in the case of grinding wheels for grooving and bevelling.

Manual renewal of grinding wheel surfaces has thus been proposed. Whenthe operator considers that wheel is blunt, he employs a stick ofalumina or carborundum or a material of similar hardness which he passesmanually over the wheel surface to renew it. There are manydisadvantages associated with this procedure:

Firstly, it leads to premature wear of the diamond surface when theresharpening operation are repeated frequently, or when the operatorperforms the operation too brutally. Even if the grinding wheel is stillworking, the operator may judge that machining is not proceeding fastenough and decide to renew the grinding wheel surface prematurely. This,bearing in mind the high cost of the grounding wheel, is a majordisadvantage.

Secondly, the manual procedure, and despite all the experience theoperator may possess, causes deformation of the periphery of thegrinding wheel. In the case of a trimming wheel, with a flat peripheralcross-section, the alumina or carborundum stick must be held exactlyparallel to the edge of the wheel to avoid deforming its profile. Theoperation is very difficult to carry out in a satisfactory way. In thecase of a bevelling or grooving wheel which has a projecting or recessedprofile, manual resharpening using an alumina or carborundum stickcauses appreciable deformation of the machining profile of the grindingwheel. Because of this, after resharpening, the wheel no longer has asuitable shape to ensure completely satisfactory machining.

German Patent Application 4,012,658 discloses a system for truing agrinding wheel applied to a machine for grinding ophthalmic lenses.According to this document, the grinding wheel is trued using a toolthat is mounted between the spindles in place of the lens. This systemdoes not however enable the problem of grinding wheel surface renewal tobe resolved. In all cases, the distance between the grinding wheel andtool axes stays practically constant and it is only possible to true theprofile, but to not renew the grinding wheel surface. Moreover, thesystem has no provisions whatsoever for detecting when the grindingwheel needs truing.

SUMMARY OF THE INVENTION

The present invention discloses a method for renewing grinding wheelsurfaces which overcomes the disadvantages of the prior art. It alsodiscloses a disk for grinding wheel surface renewal and a grindingmachine for implementing the method.

Grinding wheel surface renewal is, thanks to the present invention,rapid and reliable. It enables the machining profiles of the grindingwheel to be carefully preserved, thus leaving the tool's performanceintact.

It also prevents premature wear of the grinding wheel by allowingaccurate checking of their degree of wear to be performed.

It also allows surface renewal adapted to the various materials of whichthe wheel may be made to be carried out.

The present invention provides a method for renewing grinding wheelsurfaces in a machine for grinding ophthalmic lenses, comprising one orseveral diamond grinding wheels, gripping means designed to receive anophthalmic lens and control means, in which the method comprises a stepfor detecting wear of the grinding wheel, surface renewal being carriedout using a disk of an abrasive material fitted on the grinding machinein the place of a lens in the gripping means for said lenses.

According to one embodiment of the invention, the method comprises astep for automatically detecting the presence and nature of the disksfitted in said gripping means.

In one embodiment, the step for detecting the wear of the grinding wheelconsists in measuring the time to machine an ophthalmic lens.

In another embodiment, the step for detecting the wear of the grindingwheel consists in measuring the power consumed by the means for drivingthe diamond grinding wheel in rotation and/or the means for driving thegripping means in rotation.

The invention also provides a disk for carrying out the method, in whichat least the peripheral surface of said disk is constituted by anabrasive material, and it includes in its central portion means forproviding coupling to the said gripping means of said machine.

The disk further comprises, at its central portion, identification meansconsisting of a plurality of holes. The abrasive material advantageouslyconsists of alumina or carborundum.

The invention also provides a machine for grinding ophthalmic lenses forcarrying out the above method, comprising one or several diamondgrinding wheels, gripping means designed to receive an ophthalmic lensand control means for controlling the rotation and the relative positionof said grinding wheels and said lens, further comprising means fordetecting the wear of said grinding wheel or wheels by carrying outcomparison with a target value for the time needed to machine the lensor for the power consumed by the means for driving said grinding wheeland/or said gripping means in rotation.

In one embodiment of the invention, the machine further comprises meansfor detecting the possible presence and the nature of the disk forrenewing the surface of said grinding wheels, fitted in said grippingmeans in the place of said lens.

In a further embodiment of the invention, the detection means consist ofa photoelectric cell and a light source.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will become moreclear from the description that follows provided by way of example andwith reference to the attached drawings in which:

FIG. 1 is a diagrammatical view of the lens grinding machines;

FIG. 2 shows a disk according to the present invention;

FIG. 3 is a detailed view on a larger scale, of the profile of a diskaccording to the invention;

FIG. 4 is a flow chart of an automatic surface renewal program.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a diagrammatical view of a machine for grinding lenses; onlythose parts directly concerned by the present invention are shown inFIG. 1. Such a machine comprises, in a manner known per se, a set ofdiamond grinding wheels, gripping means designed to receive anophthalmic lens and control means for controlling the rotation and therelative position of the said grinding wheels and said lens. Only onetrain of grinding wheels 1 has been shown on FIG. 1 and this for exampleconsists of four grinding wheels grouped together consisting of a wheel2 for trimming plastics lens blanks, a wheel 3 for trimming glass lensblanks, a grooved wheel 4 for finishing and beveling, and a ribbed wheel5 for finishing and grooving. Wheel train of 1 is designed to rotate onan axis 6 driven by a motor which is not shown, controlled by controlmeans. The machine further includes gripping means consisting, forexample in FIG. 1 of two carrier spindles 7 and 8 designed to clamp alens. In FIG. 1, the situation in which disk 9 according to theinvention is mounted in the gripping means 7, 8 in place of the lens isillustrated. The complete assembly consisting of the gripping mean 7, 8and the lens or the disk 9 provided in its place is designed to rotateabout the axis 10, driven by a motor, not shown, controlled by controlmeans. The control means thus control the rotation of the grinding wheeltrain 1 about axes 6, and rotation of the complete assembly consistingof gripping means 7, 8 and disk 9 about axes 10. The control meansfurther control relative displacement of the train of grinding wheels 1of the assembly 7, 8, 9 in the direction of the axes 6 and 10 asindicated by arrow 11 on FIG. 1, and in the direction perpendicular tothe axes 6 and 10 as indicated in FIG. 1 by the arrow 12. A disk 9 witha generally plane peripheral section has been shown in FIG. 1, this diskbeing suitable for renewing the surface of cylindrical grinding wheels,such as the blank trimming wheels 2 or 3.

The method consists in fitting an abrasive material disk 9 into thegripping means 7, 8 and then renewing the surface of the grinding wheelusing said disk. For surface renewal, the disk 9 and the train ofgrinding wheels are driven in rotation and the control means ensure thedisk 9 is positioned facing the grinding wheel to be re-sharpened, byrelative displacement in the direction of the axis indicated by arrow11. Following this, the command means ensure the disk 9 and the grindingwheel move relatively one with respect to the other so that they comeinto contact for the period necessary to renew the surface of thegrinding wheel. Obviously, this time will depend on the latter'scomposition as well as on the hardness of the abrasive disk 9. Accordingto the invention, surface renewal is carried out after wear of thegrinding wheel has been detected, as will be described in more detailbelow.

The control means have not been shown in FIG. 1 as their exact manner ofoperation is not in any way essential to a full understanding of theinvention. Conventionally, they can comprise a sensing finger providedwith a sensing probe or any other suitable measuring device. The controlmeans may include a specific program ensuring good surface renewal, andwhich is for example stored in the machine in a support such as anEPROM-type programmable memory. In fact, the invention can be adapted toall type of automatic grinding machine having widely varying control andgripping means.

FIG. 2 shows a disk according to the present invention. Such a disk hasa peripheral portion 13 consisting of an abrasive material. In the caseof a diamond grinding wheel, the material can for example be alumina orcarborundum. The disk in FIG. 2 has, at its central portion 14, means 15for coupling it to the gripping means of the grinding machine in whichit is to be employed. In the case of FIG. 2, the coupling means 15 matchthe structure of the spindles 7, 8, so that the disk 9 can be adapted tobe held and driven in rotation by gripping means of the grindingmachine. The disk in FIG. 2 furthermore carries at its central portion14, identification means consisting of a plurality of holes 16a, 16b,16c allowing the various disks to be identified. Actually, as has beenseen above, grinding machines are most frequently fitted with a train ofwheels of differing hardnesses and contours, and it is necessary to use,for renewing the surface of the various grinding wheels, disks ofvarious compositions and contours. The identification means allow thevarying disks to be recognized.

In the embodiment in FIG. 2, the identification means consist of holes16a, 16b, 16c provided in the central portion of the disk on the samecircle concentric with the disk. The disk is identified by counting thenumber of holes. Counting can by be done by the operator in the casewhere he has no other means of identification available such as, forexample, labelling, color of the grinding wheel etc. Counting can alsobe done by the machine thanks to the provision of means for detectingthe presence and the nature of the grinding wheel. In the case of FIG.2, the disk comprises, as the means of identification, holes 16a, 16b,and 16c and the detection means of the grinding machine advantageouslyconsist of a light source fixed on the machine frame and a photoelectriccell disposed opposite said source. The light source and thephotoelectric cell are arranged such that when a disk is fitted onto thegrinding machine, the light source and the photoelectric cell are onopposite sides of the disk and the light beam leaving the light sourcehits the photoelectric cell after having passed through one of the holes16a, 16b, 16c. When the disk rotates, it is possible, thanks to thepresence of the source and the light cell, to count the number of lightflashes arriving at the photoelectric cell thus enabling the number ofholes on the disk to be counted. Advantageously the source is a sourceof infrared light and the cell is adapted to detect infrared so thatspurious detection due to ambient light can be avoided. Obviously, thesource and the cell are only one possible way of providing the detectingmeans which may be present on the machine. Their function can also beprovided by other means.

FIG. 3 shows detail, on a larger scale, of the contour of the disk andof a grinding wheel such as, for example, the grinding wheel 5 ofFIG. 1. On FIG. 3, a cross-section through the peripheral portion of thegrinding wheel 17 designed to carry out lens grooving, and across-section through the peripheral portion of the disk 18 for renewingthe surface, according to the invention, of this grinding wheel areshown. The grinding wheel 17 has a triangular rib 19 designed to form agroove in the lenses. The contour of the disk 18 can, for its part, beobtained starting from a disk with a plane peripheral surface, after aninitial surface renewal operation on a grinding wheel, such as grindingwheel 17, the contour of which has not become deformed. The contour ofgrinding wheel 17 and in particular, the shape of the rib 19 arepreserved when the disk 18 is used for surface renewal; the latteradditionally supplies, after surface renewal, the pattern of thegrinding wheel's contour which is thus obtained with high accuracywithout the need to remove the grinding wheel. The pattern enables theoptician to determine the presence of defects in the grinding wheel,using comparison with the theoretical contour to be obtained.

FIG. 4 is a flow chart showing the steps in the renewal method accordingto the invention. The flow chart in FIG. 4 is obviously only one exampleenabling the possible uses of the various elements of the invention tobe clearly understood.

The machine according to the invention comprises means for detecting thewear of a grinding wheel. One example of an embodiment of said means isexplained in detail with respect to the first two steps in the flowchart in FIG. 4.

At step 20, the machine measures the time needed to machine the lens. Itis possible to measure the total time for the trimming andgrooving/beveling operations or to measure each one of the durationsindependently. The duration is measured either using the internal clockof the microprocessor or using an external clock read by themicroprocessor. The external clock can for example consist of quartzclock means.

At step 21, the measured duration is compared with the target valuecorresponding to the mean machining time for a worn grinding wheel. Ifthe measured duration is less than the target value, the grinding wheeldoes not need re-sharpening and the machine is available for the nextmachining operation. If, however, the measured duration is above thetarget value, the grinding wheel has taken an abnormally long time tomachine the lens and its surface can be considered as having becomedull. Surface renewal can then start, this occurring at step 22. If thisis not necessary, the control returns to step 20.

At step 22, the grinding machine displays a message indicating the needfor surface renewal, for example through flashing of a lamp, or bydisplay on a liquid crystal display or by any other means.

At step 23, the machine, thanks to the detection means incorporatedtherein, and the means for identifying the disk, detect the presence andthe nature of said disk mounted in the gripping mean.

At step 24, it is determined whether the disk fitted in the grippingmeans is a suitable one. If this is not the case, control returns tostep 22. If the disk is suitable, control proceeds to step 25. At step25, the disk is positioned opposite the grinding wheel to bere-sharpened by a movement parallel to the axis of the grinding wheel asindicated by arrow 11 in FIG. 1.

At the next step 26, the grinding wheel and the disk are rotated andcome into contact for the time needed to ensure surface renewal. Again,the period of time depends on the type of grinding wheel as well as onthe nature of the disk and its possible wear.

The program then returns, after the grinding wheel surface has beenrenewed, to its starting point the machine being ready for use again.

The program shown in the flow chart of FIG. 4 is obviously able to bemodified; thus, simultaneous re-sharpening of certain grinding wheelscould be provided for, or any other variation thereon.

The wear on the grinding wheel can be detected, according to theinvention, by means other than measuring the time needed to machine alens, described above with reference to FIG. 4. Thus, one can alsomeasure the power absorbed by the various motors during a lens grindingoperation. The power is measured at step 20 of the flow chart in FIG. 4instead of, or simultaneously with, measurement of machining time. Thepower consumed by the motor driving the grinding wheel in rotation anddriving the gripping means 7, 8 in rotation is measured, the measurementbeing for example achieved through the use of voltage and/or currentdetermining means for the drive motor.

At step 21 in the flow chart of FIG. 4, this measured power value iscompared with a target value. As described above, depending on theresult of the comparison, control either passes to step 22 or returns tostep 20, optionally after a brief wait.

The power consumed by the motor during lens grinding allows wear of thewheel to be determined. When the grinding wheel has lost some of itsabrasive power, the frictional forces between the wheel and the lensbeing ground increase and the torque the motor needs to supply increasescorrespondingly. Thus, wear of the grinding wheel is detected.

The power measurement can be one actual value or it can also be a meanvalue for a given number of lenses.

Wear of the grinding wheel can thus be measured.

Obviously, the prevent invention is not limited to the embodimentsdescribed and illustrated but may undergo various modificationsavailable to those skilled in the art without this leading to adeparture from the scope of the invention. In particular, the inventionhas been described for the very particular case of machines for grindingophthalmic lenses. It is obvious that it can be adapted to a pluralityof other types of grinding machines.

What is claimed is:
 1. A method for renewing grinding wheel surfaces ina machine for grinding ophthalmic lenses, comprising one or severaldiamond grinding wheels, gripping means designed to receive anophthalmic lens and control means, and comprising a step for detectingwhether a surface of the grinding wheel needs renewing, and in whichsurface renewal is carried out using a disk of an abrasive materialfitted on the grinding machine in the place of a lens in said grippingmeans for said lenses, further comprising a step for automaticallydetecting the presence and the composition and contour of disks fittedin said gripping means.
 2. Method according to claim 1, wherein the stepfor detecting whether the surface of the grinding wheel needs renewingconsists in measuring the time to machine an ophthalmic lens.
 3. Methodaccording to claim 1, wherein the step for detecting whether the surfaceof the grinding wheel needs renewing consists in measuring the powerconsumed by the means for driving the diamond grinding wheel inrotation.
 4. Method according to claim 1, wherein the step for detectingwhether the surface of the grinding wheel needs renewing consists inmeasuring the power consumed by the means for driving the gripping meansin rotation.
 5. Method according to claim 1, wherein the step fordetecting whether the surface of the grinding wheel needs renewingconsists in measuring the power consumed by the means for driving thediamond grinding wheel in rotation and by the means for driving thegripping means in rotation.
 6. A method for renewing grinding wheelsurfaces in a machine for grinding ophthalmic lenses, comprising one orseveral diamond grinding wheels, gripping means designed to receive anophthalmic lens and control means, and comprising a step for detectingwhether a surface of the grinding wheel needs renewing, and in whichsurface renewal is carried out using a disk of an abrasive materialfitted on the grinding machine in the place of a lens in said grippingmeans for said lenses wherein the step for detecting whether the surfaceof the grinding wheel needs renewing consists in measuring the time tomachine an ophthalmic lens.
 7. A method for renewing grinding wheelsurfaces in a machine for grinding ophthalmic lenses, comprising one orseveral diamond grinding wheels, gripping means designed to receive anophthalmic lens and control means, and comprising a step for detectingwhether a surface of the grinding wheel needs renewing, and in whichsurface renewal is carried out using a disk of an abrasive materialfitted on the grinding machine in the place of a lens in said grippingmeans for said lenses, wherein the step for detecting whether thesurface of the grinding wheel needs renewing consists in measuring thepower consumed by the means for driving the diamond grinding wheel inrotation.
 8. A method for renewing grinding wheel surfaces in a machinefor grinding ophthalmic lenses, comprising one or several diamondgrinding wheels, gripping means designed to receive an ophthalmic lensand control means, and comprising a step for detecting whether a surfaceof the gripping wheel needs renewing, and in which surface renewal iscarried out using a disk of an abrasive material fitted on the grindingmachine in the place of a lens in said gripping means for said lenses,wherein the step for detecting whether the surface of the grinding wheelneeds renewing consists in measuring the power consumed by the means fordriving the gripping means in rotation.
 9. A method for renewinggrinding wheel surfaces in a machine for grinding ophthalmic lenses,comprising one or several diamond grinding wheels, gripping meansdesigned to receive an ophthalmic lens and control means, and comprisinga step for detecting whether a surface of the grinding wheel needsrenewing, and in which surface renewal is carried out using a disk of anabrasive material fitted on the grinding machine in the place of a lensin said gripping means for said lenses, wherein the step for detectingwhether the surface of the grinding wheel needs renewing consists inmeasuring the power consumed by the means for driving the diamondgrinding wheel in rotation and by the means for driving the grippingmeans in rotation.
 10. A disk for carrying out a method for renewinggrinding wheel surfaces in a machine for grinding ophthalmic lenses,comprising one or several diamond grinding wheels, gripping meansdesigned to receive an ophthalmic lens and control means, and comprisinga step for detecting whether the surface of the grinding wheel needsrenewing, and in which surface renewal is carried out using a disk of anabrasive material fitted on the grinding machine in the place of a lensin said gripping means for said lenses, wherein at least the peripheralsurface of said disk is constituted by an abrasive material, said diskincluding in its central portion means for providing coupling to thesaid gripping means of said machine, wherein it further comprises, atits central portion, identification means consisting of a plurality ofholes.
 11. The disk according to claim 10, wherein said abrasivematerials consists of alumina or carborundum.
 12. A disk for carryingout the method for renewing grinding wheel surfaces in a machine forgrinding ophthalmic lenses, comprising one or several diamond grindingwheels, gripping means designed to receive an ophthalmic lens andcontrol means, and comprising a step for detecting whether the surfaceof the grinding wheel needs renewing, and in which surface renewal iscarried out using a disk of an abrasive material fitted on the grindingmachine in the place of a lens in said gripping means for said lenses,wherein at least the peripheral surface of said disk is constituted byan abrasive material, said disk including in its central portion meansfor providing coupling to the said gripping means of said machine,wherein said abrasive materials consist of alumina or carborundum.
 13. Amachine for grinding ophthalmic lenses for renewing grinding wheelsurfaces, comprising one or several diamond grinding wheels, grippingmeans designed to receive an ophthalmic lens and control means forcontrolling the rotation and the relative position of said grindingwheels and said lens, wherein it further comprises means for detectingwhether the surface of the grinding wheel needs renewing or wheels bycarrying out comparison with a target value for the time needed tomachine the lens or for the power consumed by the means for driving saidgrinding wheel and said gripping means in rotation.
 14. The machineaccording to claim 13, wherein it further comprises means for detectingthe possible presence and the composition and contour of the disk forrenewing the surface of said grinding wheels, fitted in said grippingmeans in the place of said lens.
 15. The machine according to claim 14,wherein said detection means consists of a photoelectric cell and alight source.